A disadvantage of CMOS image sensors is its sensitivity to strong light when it shines onto the pixel array. A typical example of this problem is when the camera is pointed directly to the sun; the sensor will output a “black” sun instead of a bright one in the image. This phenomenon is referred to as eclipse or darkle. U.S. Ser. No. 10/607,943, filed Jun. 27, 2003, entitled “CMOS Image Sensor Oversaturation Protection Circuit,” by Christina Phan et al., addresses this problem. Referring to FIG. 1, there is shown a pixel array 10, column and eclipse circuit 20, and a correlated double sampler circuit 30. This prior art technique requires the correction to be implemented in the column and eclipse circuit 20. As the pixel pitch gets smaller, it may not be feasible to fit a single column circuit in the width of a single pixel. Therefore, it is common to put multiple columns together as a unit cell. The entire column-circuit array is then generated by replicating this unit cell. In this case, including the correction feature will make the layout of the column circuit extremely challenging. The imbalance created by including this feature will worsen the mismatch between the columns. And as a result of this, a pattern of structured noise, measured as Fixed Pattern Noise (FPN), an important optical specification in image sensors, will be present in the image.
Consequently, a need exists for overcoming this shortcoming. The present invention addresses this shortcoming by permitting the correction to be done as a stand-alone circuit. This preserves the balance of the layout of the column circuit while providing a solution to the eclipse or darkle problem.